Circuit design engineers within the dynamic electronics industry are constantly looking for components that are smaller and more efficient. Ever-increasing device densities have been driven by demand for ever-increasingly capable products that require more and more memory to support ever more complex circuitry for ever faster process and more capable control systems.
A basic active circuit design element is the Field Effect Transistor (FET), a voltage-controlled semiconductor device in which the output current is controlled by the voltage imposed at an input gate. FETs are used effectively for a wide range of applications, from high-speed digital logic and high-power analog circuits to low current analog, Radio Frequency (RF) and mixed signal (MS) circuits. With their very high input impedance, FETs are useful for amplification and processing of very low current signals in battery-powered devices.
A FET may be configured as a three electrode device with a drain, a source, and a gate electrode, or as a multi-electrode device where two or more gate electrodes are incorporated to provide a multi-gated device. In all instances, drain and source electrodes couple to a semiconducting channel, while gate electrodes control the electron flow through the channel to provide current amplification and switching capability. The gate may be constructed in different ways to achieve different device characteristics. In a Metal Oxide Semiconductor (MOS) FET, for instance, voltage control by the gate is achieved by exerting a field effect on the channel through a thin gate oxide. FETs that use a simple pn-junction (JFET) or a direct Metal-Semiconductor (MESFET), Schottky, interface have no such oxide separating the gate from the channel. MOSFETs, in particular, are found in virtually every application of microelectronics and integrated circuits, especially in the form of Complementary MOS (CMOS) devices, which are often selected for their low power consumption.
A key concern in microelectronics is being able to generate smaller devices that consume less power. Although significant progress has been made in this area, a common problem is that the influence of gate voltage on the channel diminishes as the device decreases in size and secondary effects become more prominent. Since gate voltage control is achieved by exerting a field effect on the channel, short-channel effects become more problematic as the overall transistor size decreases, and interfere with the ability of the gate voltage to maintain exclusive control over the channel. Ideally, control of the channel should be a function only of the gate voltage.
For a general purpose transistor architecture it is advantageous to offer the ability to provide different electrical configurations and threshold voltages that can be matched to specific applications. Typically, this is achieved with additional process complexity that requires more masking steps, multiple threshold implants and perhaps multiple thicknesses of gate oxide. It would be advantageous to achieve multiple transistor configurations by manipulation of the connections of the device to result in different device characteristics in order to trade power for speed or vice versa. Fabrication would be greatly simplified if the MOS gate oxide could be eliminated and various electrical behaviors could be achieved by simply modifying connections to or within the transistor cell. It would further be advantageous to provide such configurations without a penalty to the device layout area.
For these reasons among others, it would be an advancement in the art to provide a transistor architecture that offers independent multiple gate electrodes, where the superior control of the channel can be used to render smaller, more efficient analog, digital and mixed signal circuits, and where the flexibility provided by multiple electrical configurations avoids the need for modifications to the fabrication process. Such devices are disclosed and claimed herein.